1. Field of the Invention
The present invention relates to a power supply apparatus for supplying a predetermined DC voltage to an electronic tube that is used to amplify and oscillate a high-frequency signal, and a high-frequency circuit system which incorporates such a power supply apparatus.
2. Description of the Related Art
Travelling-wave tubes and klystrons are electron tubes for amplifying and oscillating a high-frequency signal based on an interaction between an electron beam emitted from an electron gun and a high-frequency circuit. As shown in FIG. 1 of the accompanying drawings, traveling-wave tube 1 has electron gun 10 for emitting electron beam 50, helix 20 serving as a high-frequency circuit for causing electron beam 50 emitted from electron gun 10 and a high-frequency signal (microwave) to interact with each other, collector electrode 30 for trapping electron beam 50 output from helix 20, and anode electrode 40 for drawing electrons from electron gun 10 and guiding electron beam 50 emitted from electron gun 10 into helix 20.
Electron gun 10 has cathode electrode 11 for emitting negative thermions, heater 12 for applying thermal energy to cathode electrode 11 to emit negative thermions therefrom, and Wehnelt cathode 13 for focusing emitted electrons into electron beam 50.
Electron beam 50 emitted from electron gun 10 is accelerated by the potential difference between cathode electrode 11 and helix 20 and introduced into helix 20. Electron beam 50 travels in helix 20 while interacting with the high-frequency signal input to helix 20. Electron beam 50 that is output from helix 20 is trapped by collector electrode 30. At this time, helix 20 outputs a high-frequency signal that has been amplified by an interaction with electron beam 50.
As shown in FIG. 1, the electrons of traveling-wave tube 1 are supplied with predetermined power supply voltages from power supply apparatus 70. Power supply apparatus 70 has helix power supply 71 for supplying a DC voltage (helix voltage Ehel), which is negative with respect to the potential of helix 20, to cathode electrode 11, collector power supply 72 for supplying a DC voltage (collector voltage Ecol), which is positive with respect to the potential of cathode electrode 11, to collector electrode 30, anode electrode 73 for supplying a DC voltage (anode voltage Ea), which is positive with respect to the potential of cathode electrode 11, to anode electrode 40, and heater power supply 74 for supplying a heater voltage Eheat, which is an AC voltage or a DC voltage with respect to the potential of cathode electrode 11, to heater 12 of electron gun 10. Helix 20 is normally connected to the case of traveling-wave tube 1 and grounded.
As shown in FIG. 2 of the accompanying drawings, helix power supply 71 comprises rectifying circuit 102 for rectifying an AC voltage output from the secondary winding of transformer 101, series regulator 103 for generating the helix voltage Ehel from an output voltage (DC voltage) of rectifying circuit 102, and capacitor bank 104 having rectifying capacitors for stabilizing the helix voltage Ehel. The primary winding of transformer 101 is connected to a known inverter, not shown, and supplied with an AC voltage therefrom.
Traveling-wave tube 1 shown in FIG. 1 is capable of controlling the amount of electrons emitted from cathode electrode 11 with the anode voltage Ea applied to anode electrode 40. Therefore, the electric power of the high-frequency signal output from traveling-wave tube 1 can be controlled by anode voltage Ea. For example, even while a high-frequency signal of constant electric power is being input to traveling-wave tube 1, traveling-wave tube 1 can output a pulsed high-frequency signal by applying a pulsed voltage to anode electrode 40.
An arrangement for controlling the high-frequency signal output from traveling-wave tube 1 with anode voltage Ea is disclosed in Japanese Patent Laid-Open No. 2005-45478, for example. Japanese Patent Laid-Open No. 2005-45478 reveals a circuit whose electric power efficiency is increased by detecting an input signal (high-frequency signal) applied to traveling-wave tube 1 and controlling the anode voltage Ea so that the output electric power will not be saturated, depending on the input electric power.
The helix voltage applied to traveling-wave tube 1 is normally a high DC voltage ranging from several hundreds V to several kV. Therefore, as shown in FIG. 2, conventional power supply apparatus 70 employs a plurality of series-connected transistors in series regulator 103 for reducing the voltage applied to each of the transistors.
Series regulator 103 shown in FIG. 2 is supplied with an input DC voltage which is output from rectifying circuit 102 and which is higher than the helix voltage Ehel. The collector-to-emitter voltage of each of the transistors of series regulator 3 is regulated to stabilize the output voltage of the power supply apparatus, i.e., the power supply voltage (helix voltage Ehel).
However, series regulator 103 shown in FIG. 2 has a relatively large output impedance value because the power supply voltage (helix voltage Ehel) is output through the series-connected transistors. Furthermore, as the time constant is large while series regulator 103 is in operation, series regulator 103 is unable to act upon load variations in times ranging from several μsec. to several msec.
Specifically, the power supply apparatus has series regulator 103 for supplying a power supply voltage through the series-connected transistors. When the power supply apparatus applies a pulsed voltage to anode electrode 40, for example, to bring traveling-wave tube 1 into pulsed operation, capacitor bank 104 discharges an abrupt energy depending on a load variation due to the pulsed operation. The voltage control operation of series regulator 103 is unable to follow the abrupt energy discharged from capacitor bank 104, resulting in a large drop of the power supply voltage (helix voltage Ehel) as the output voltage.
In order to avoid the above problem, the conventional power supply apparatus has reduced the drop of the power supply voltage by employing a large capacitance value for capacitor bank 104. As a result, the conventional power supply apparatus has suffered another problem, i.e., a large circuit scale.
Since the helix voltage Ehel is a DC voltage which is negative with respect to the potential of helix 20, as described above, the drop of the helix voltage Ehel means that the helix voltage Ehel approaches the ground potential (0 V). A load refers to the resistive component of each of the various electrodes of the traveling-wave tube that is connected to the output terminals of the power supply apparatus. For example, the load of helix power supply 71 refers to a resistive component between cathode electrode 11 and helix 20.